Art. XIV.—Notes on the Geology of Great Barrier Island, New Zealand.
[Read before the Auckland Institute, 15th December, 1920; received by Editor, 31st December, 1920; issued separately, 27th June, 1921.]
In 1919 the writer spent a short holiday at the northern end of Great Barrier Island, and found that the geology of that part of the island is rather indifferently represented by Hutton's paper of 1869,* which still remains the only important account of that area. In many respects Hutton's work is admirable, for considerable difficulties attach even now to a close study of some parts of the island, and years ago these must have been even greater. Hutton's chief error is that he failed to recognize a large area of rhyolitic rocks as such, and mapped them as “pinkish slates.” This is, however, entirely excusable, for these rocks are very finely banded, and in section resemble very finely granular sediments, though upon examination with a first-class instrument they can be seen to be in reality minutely microspherulitic rhyolites.
Park (1897), who deals with the geology of the central portion of the island in his paper on the geology and veins of the Hauraki goldfields, makes a similar error in classing them as banded sinters similar to those he asserts form the higher portions of a mountain-mass near Whanga-parapara, with a remarkable series of breakaway cliffs which give it its local name, Whitecliffs Range. (Plate XXIII, fig. 2.) The Maori name for it is Te Ahumata.
The writer's visit served to yield him little more than an approximate idea of the geology: one or two large areas to the north are terra incognita to him, and the appended map shows very crudely sketched boundaries between the various rock formations. He managed, however, to spend a day or so at Mine Bay, on the north-west coast, where Hutton maps so many interesting dykes intrusive into the shales and greywackes which form the basement of the island, and to make a moderately careful study of many of these dykes. The number of them is so great at Mine Bay itself, along the coast both north and south from there, and in the valley of Mine Bay Creek, that a full collection was out of the question, and no map could exhibit their location unless published on a very large scale.
The writer made his headquarters at the house of Mr. Warren, of Port Fitzroy, and cannot sufficiently thank Mr. Warren and all members of his household for the assistance they gave him in numerous ways. From there he made a number of excursions on foot and by boat, and finally took a walk along the recognized foot route from near Cooper's to the top of Mount Hobson, thence by a devious traverse to Awana Flat,
[Footnote] * Full reference is appended in a list of literature cited to be found at the end of this paper.
from which place he visited Oroville, in the Kaitoke Valley, and then returned by the road and tracks along the east coast from Kaitoke Creek to Harataonga Bay, whence he took the bush track, crossing the higher country some distance from the coast, to Whangapoua.
In spite of the cursory nature of the writer's examination of a large portion of the area mapped, it appeared to him desirable to make such provisional alterations and additions to Hutton's account of its geology as are now possible, instead of waiting perhaps a long time until an opportunity presented itself for making a more thorough geological survey.
Scheme of Paper.
The aim of this paper on the geology of Great Barrier Island may be summarized as follows:—
To present a statement of the physiography and stratigraphy of the island, and more particularly of its northern half:
To record the discovery in the basement (? Mesozoic) rocks of some interesting conglomerate bands containing granite, pegmatite, granulite, and other boulders:
To describe a little more fully than Hutton (1869) the rocks intrusive into the basement:
To discuss briefly the origin of the copper lode at Miner's Head.
Physiography of Great Barrier Island.
Great Barrier Island is a rugged, elevated, much-dissected, probably one-cycle mountain-mass, about twenty-four miles in length, and varying up to thirteen miles in width. It is built largely of moderately resistant rocks, amongst which well-compacted andesitic conglomerates and breccias and rhyolite lavas figure most prominently. Each of these two rock-types builds its own characteristic terrain, recognizable with ease even at considerable distance. The andesitic fragmentals often build the hill land-scape best described as turreted, with successions of frowning bluffs breaking the monotony of gentler slopes. The rhyolites lend themselves to the evolution of the weirdest pinnacled crags and sheer precipices, which, with alluring whiteness, give a fascinating picturesqueness to the landscape carved from them. (See Plate XXIV.)
The area of shales and greywackes at the north of the island lacks much of the ruggedness of the more southerly portion, but is none the less steep and topographically fine-textured. On the north-west it descends abruptly to the sea in stupendous lofty precipices. (See Plate XXIII, fig. 1.)
Like its prototype the Cape Colville (or Coromandel, or Hauraki) Peninsula, of which it is undoubtedly the former continuation, Great Barrier Island represents the remnant of a maturely dissected, mountainous, heterogeneous land-mass with insequent drainage, which was depressed with reference to sea-level in the not-far-distant geologic past, so that the sea entered far into the deep, comparatively narrow trenches carved in the earlier mass.
More particularly on the western coast, islets and reefs thickly fringe the shore-line, representing extensions of this earlier land-mass which have not yet been cut down by wave-attack. (See Plate XXII, fig. 1.) Youthful, precipitous, lofty cliffs form this highly irregular immature coast, except locally where bays such as Katherine, Blind, and Tryphena Bays exhibit
unimportant shore progradation, or where at Port Fitzroy the cliffs are interrupted temporarily by deep, narrow entrances to the wonderful and beautiful harbour.
Delta - building is active in bay - heads entered by streams of any importance, but there is a noteworthy absence, even in the landlocked Fitzroy Harbour, of the mangrove-dotted mud-flats so common in most of the North Auckland harbours. This is to be accounted for in part because of the insignificant size of the inflowing streams, in part because of the great depth of the sea-occupied trenches.
The eastern coast of the island differs very greatly from the western. It is exposed to more vigorous wave-attack from the ocean, with the result that it has been cut back until the coast-line is much more regular than the western. Several large harbours similar to Port Fitzroy existed at one time, but all have been shut off from the open sea by spits or barrier beaches, and the resulting lagoons have in large part been obliterated by blown sand and swamp or other filling. One of the best examples is furnished by the lower Kaitoke area, in the central portion of the island The earlier inlet has apparently been enclosed by a barrier beach. Landwards from this is a zone of low sand-dunes, and then comes a remarkable area of swamps. (See Plate XXII, fig. 2.) At the Awana Stream, similarly, swamps occupy an extensive tract within a flaring portion of the lower valley, just above a bottle-necked outlet to the ocean which is due to the close approach of two opposed spurs cut in resistant andesitic fragmentals.
Barrier Beaches or Spits.
In considering whether the former harbours of the eastern coast of Great Barrier Island have been blocked off by barrier beaches or spits one has many opposing considerations to weigh. The problem is best considered by reference to the analogous physiographic conditions of the Cape Colville Peninsula, where, in similar manner, the western harbours remain open, whilst the eastern are largely shut off by wave-built sand-accumulations.
There is undoubtedly a strong northward drift of the sea-waters, which brings pumice, for example, from the Bay of Plenty around Cape Colville and deposits it in such places as Whangateau (near Cape Rodney) on the shores of the mainland; but this cannot have had any effect in creating the present conditions at Great Barrier Island, for both coasts should show similar features if this were so.
The fundamental reason undoubtedly is that which has allowed the building of such typical barrier beaches as the somewhat complex one that encloses the Katikati—Tauranga harbour, on the southward continuation of the east coast of Coromandel Peninsula. There is abundant evidence that the waves of the open ocean have in that district removed a very considerable strip of land in cutting back the present sea-cliffs.
It is also to be observed that the depth of water off shore at the conclusion of the major movement of subsidence noted was shallow wherever barrier beaches have been built, for in such localities the earlier land-surface was invariably of low relief, often consisting of the flood-plains of the now greatly diminished rivers, or of the flattish floors of their wide, late-mature valleys. The initiation of the building of the beaches is almost certainly to be correlated with sub-recent uplift of a few feet, which is demonstrated by uplifted shore-terraces, wave-cut platforms, sea-caves,
Fig. 1.—The rugged, youthful, western coast of Great Barrier Island. View looking north-north-west towards the entrance to Port Abercrombie. The sea-cliffs are cut in andesitic fragmentals.
Fig. 2.—Kaitoke Beach from the south, mid-east coast, Great Barrier Island. Mount Hobson is visible in the centre-right distance.
and other similar criteria observable at different portions of the coastlines in the mid-Auckland area; for, as D. W. Johnson shows in Shore Processes and Shore-line Development, the disturbance of the equilibrium of the graded off-shore profile by uplift is the most general cause of the building of these off-shore bars.
In corresponding manner, at the Great Barrier Island, a graded profile must have been established fairly early by vigorous wave-attack upon the easily removed areas of low relief on the eastern coast, and barrier beaches would soon come into existence upon any subsequent uplift taking place.
Summary of Stratigraphy.
Great Barrier Island is constituted by a basement mass of folded sediments, largely shales and greywackes, and herein called the “oldermass,”* which extend over the northern part of the island as far south approximately as a line drawn from the head of Katherine Bay on the west coast to Tupawai on the east, and which are again exposed in a small area near Harataonga Bay, farther south. These rocks have been extensively eroded and then covered in the Tertiary by a sheet of andesitic volcanics which is probably well over 1,000 ft. in depth. These are in turn overlain by later acid volcanic rocks, with accompanying sinters, in a central area around and south of Mount Hobson.
The andesitic rocks are largely coarse fragmentals, with subsidiary lavas; they form the mass of the island south of the northern sedimentary area, and are covered at higher levels by acidic rocks in the area mentioned.
The Basement Sediments, or Oldermass.
The writer closely examined the outcrops of the older sediments for fossils, but was unable to find any, in spite of the fact that (fide Sollas and McKay, 1905, vol. 1, p. 146) Hutton discovered a coral. There is little doubt that the oldermass of the island is comprised of rocks substantially the same as those of Coromandel Peninsula which yielded a few Mesozoic fossils south of Coromandel (Fraser and Adams, 1907, pp. 49–50). In facies they are mainly shales, but with moderately frequent greywackes which are sometimes—as, for example, at Harataonga Bay—finely interbanded with the shales. In the same locality, further, a small amount of fine conglomerate is displayed, which recalls somewhat the conglomerate of the comparable Manaia series of Coromandel Peninsula (Fraser and Adams, 1907, pp. 48–62). In the headwaters of Mine Bay Creek there is a coarse conglomerate with greywacke boulders.
One of the most striking features of the oldermass is the way in which its rocks have been seamed by the numerous dykes mentioned in the introduction above. A detailed account, of their petrography will be given in a later section
A most interesting and important discovery was made of bands of coarse conglomerate with abundant granitic, pegmatitic, and granulitic pebbles and boulders. Undoubtedly these yield very definite information as to the character of the earlier (pre-Mesozoic) land-mass affording the clastic material.
[Footnote] * A usage introduced to New Zealand geology by Cotton (1916).
The structure of the oldermass in the northern portion of New Zealand is still imperfectly known, so that a few definite observations of strike and dip may prove of value. In the vicinity of the intrusive dykes intense shattering has disguised the structure of the sediments. Plate XXV, fig. 3, illustrates folding in these beds at Harataoaga Bay.
Observations.—(1.) Immediately south of Miner's Head a conglomerate band strikes N. 35° W., with a dip of 70° to the north-east. (2.) In the valley of Mine Bay Creek two conglomerate bands gave respectively (a) strike north-west and south-east, dip 60° to the south-west; (b) strike N. 60° W., dip 70° to the south-south-west. (3.) Towards the head of Mine Bay Creek: strike N 5° W., dip 30° to the east.
Conglomerate Bands in Basement Sediments.
Three outcrops of conglomerates were found—the first at the foreshore near the adit crosscut of the old copper-mine at Miner's Head, on the northwest coast of the island; the other two not far distant in branches of a small tributary to Mine Bay Creek, which enters from the south about half a mile up-stream from the foreshore. The first varies in width from about 8 in. to a little over 1 ft., and contains large well-rounded beach-boulders ranging in size up to 10 in. in diameter. The material of the boulders is typical coarse granite with conspicuous white mica, a biotite granite with equally conspicuous biotite, and plentiful hard shales and other sedimentary types.
The outcrops in Mine Bay Creek basin show a much more substantial depth than the first mentioned; both probably belong to the same band, which has a width of about 7 ft. The majority of the boulders are much smaller and less assorted than in the other band, and there is an abundance of arkositic matrix. A bi-mica granite, in boulders as large as 18 in. in diameter, forms the bulk of the constituent boulders, but shales too are plentiful, whilst granulites (some with garnet, some without), pegmatites, and occasional andesite are also represented.
Sections were cut from a number of the boulders, but microscopic examination did not add greatly to the knowledge gained by macroscopical examination. One fact worth mention is that the biotite of some of the boulders from the band near the copper-mine adit contains small zircon crystals around which are intense pleochroic haloes.
The pegmatites are fine-grained, composed almost wholly of graphically intergrown orthoclase and quartz, with frequent small flakes of biotite. The photomicrographs, figs. 1, 2, and 3 of Plate XXVII, adequately exemplify a pegmatite and two types of granulite, one with garnet and the other lacking it.
Significance of the Material of the Conglomerates.
The presence of rocks such as granulites in the basement shales and greywackes of Great Barrier Island indicates the existence near that area of a land-mass which had been subjected to intense pressure before the deposition of those sediments, a question already considered in some detail by the writer in a recent paper (Bartrum, 1920). The coarse, well-rounded nature of the boulders of the conglomerates, and their freshness, particularly in the band near Miner's Head, indicate that they were deposited near the shore-line of a land-mass. They suggest a temporary movement of elevation
of the area of deposition, followed by a continuance of depression. There is, however, no evidence to show the exact location of the land-mass, but we are undoubtedly beginning to know a little more of it than previously. It certainly lies buried beneath the unmetamorphosed sediments of the Whangarei district, for andesitic rocks intrusive into these sediments at Parua Bay contain very abundant xenolites of hornblende-schists and hornblende-epidote-schists.
The period of pressure causing this acute metamorphism of the rocks of this pre-Mesozoic land probably was coeval with that causing the granulation of granites in the central portion of the North Island (Park, 1893), and of dioritic rocks at Albany, near Auckland (Bartrum, 1920), now found in Tertiary conglomerates in those districts. It is a fair inference that this land was extensive both north and south of the city of Auckland.
The Andesitic Volcanic Bocks.
The andesitic mass resting upon the basement of eroded sediments, and occupying the main portion of the island south of the northern area of sediments, consists to a great extent of coarse fragmentals, breccias in the main, though conglomerates are also abundant. Many of the rocks here loosely called breccias are perhaps more strictly agglomerates, but the writer had not an opportunity in the field of making the distinction. Lavas of limited extent are frequently intercalated in the mass, but tuffs are scarce. Hutton (1869) records the presence of seams of black laminated shale in a coarse soft tufaceous sandstone forming the base of the series at Onewhero, in Maori (Katherine) Bay, a locality not visited by the writer.
Sollas and McKay (1905, vol. 1, p. 146) describe a hyalopilitic pyroxene-andesite belonging to this series of rocks, and Park (1897), though he does not definitely state that the propylites, or altered andesites, of the central portion of the island, which carry gold-silver veins, belong to the series, leads one to infer that he believes such to be the case, and records types that “are augitic and generally contain hypersthene, which often occurs in excess of the augite.”
A number of sections were cut from flows in many diverse localities, and of some of the fragmental material. All indicate a remarkable uniformity of facies. Hypersthene-andesites are very common, augite in these being greatly subordinate to the hypersthene, or even absent. In one slide the hypersthene has deep resorption borders of iron-ore, which is not at all a common phenomenon; this is well illustrated by the photomicrograph, fig. 4 of Plate XXVI. The other varieties of andesite can be classed as pyroxene types, with both augite and hypersthene prominent.
In the majority of the sections there is surprising uniformity in general appearance. The clear-cut phenocrysts, embracing always plentiful feldspar in addition to pyroxene, are spread in a very constant minutely crystalline groundmass consisting mainly of tiny feldspar laths with a little pyroxene and iron-ore. Sometimes it is so fine as to be practically irresolvable, and in such cases it is perhaps to be considered hyalopilitic.
A boulder from a conglomerate at Port Fitzroy furnishes a good example of intersertal structure: numerous small crystals of plagioclase, with other coarser crystals of the same mineral and pyroxene, are interspersed closely in a glass crowded with minute prisms of pyroxene and a few small crystals of magnetite. Fig. 5 of Plate XXVI illustrates a typical portion of a section of pyroxene-andesite.
Comparison with similar Fragmental Series.
There is little donbt that the andesitic mass of Great Barrier Island is coeval with that so well exhibited in rocks possessing similar mode of occurrence at Coromandel and elsewhere throughout the Coromandel Peninsula. These are the “Beeson's Island” or “second period” rocks of earlier writers (cf. Fraser and Adams, 1907; Fraser, 1910), which are considered Miocene in age.
Acidic Volcanic Rocks of the “Third Period.”
In conformity with conditions on the Coromandel Peninsula, where the latest volcanics are practically without exception rhyolitic lavas, breccias, and tuffs, and cap an erosion-surface of the “second period” andesites, there are on Great Barrier Island truly comparable acidic “third period” rocks. The writer did not make as extensive an examination of them as he desired, but visited them on the lower north-west slopes of Mount Young, the western slopes of Mount Hobson, and examined them fairly thoroughly along the ridges south-east and east of Mount Hobson which form the long pinnacled divide between the headwaters of Kaitoke Stream and of Awana Stream. The writer's route from the top of Mount Hobson was an irregular zigzag along this ridge, and across the upper basin of the Awana Stream to Awana Flat. Along his route, and particularly west of it in the circle of precipitous crags surrounding a basin-like hollow in the headwaters of the Kaitoke Stream, these “third period” rocks are by far the most conspicuous feature of the landscape, and the bizarre pinnacles and sheerwalled bluffs are scarcely to be matched even in rugged regions such as that of exactly similar rocks on the main divide between the Kauaeranga and Tairua Rivers, south-east of Thames. (See Plate XXIV.)
The main portion of the mass of acidic volcanic rocks at Great Barrier Island appears to consist of pinkish-grey rhyolitic lava with very fine wavy fluxion-banding. There is breccia in several places, but it is apparently not extensive. No important tufaceous beds were seen, but the topography in the vicinity of Mount Young indicates their possible occurrence there in quantity.
Hutton (1869) and Park (1897) deal with the rocks of this series, the former excusably considering them “pink slates,” the latter bedded sinter. Undoubtedly Park was influenced by the occurrence of siliceous sinter in large quantity on the Whitecliffs Range; McKay's able description leaves no room for doubt that there is here a considerable mass of sinter (McKay, 1897) Park (1897) refers to the same mass, and adds an interesting detail, which the writer can verify from examination of specimens given him, to the effect that some of this sinter is oolitic. The oolites are about ⅛ in. in diameter, and consist of concentric shells of very fine white mud; they imperfectly resemble the celebrated oolites of the Carlsbad springs. A few sections were cut from rocks forming the fringe of the Whitecliffs (or Te Ahumata) mass, and tend to show that all of it is not sinter, but more or less silicified rhyolite and rhyolitic tufaceous breccia. The silicification is perhaps a result of the hydrothermal activity manifested by sinter in other parts of the mass; but this suggestion is at best a surmise.
Park (1897, p. 105) refers in some detail to “a remarkable breccia, which has been carved by subaerial denudation into the most fantastic and grotesque features,” which, he says, is “immediately east of the great
siliceous deposit” (? rhyolite), and forms the watershed between the Kaitoke and Awana Streams. At every point where the writer examined the rocks at this ridge they were of the usual rhyolite, but he admits that certain portions of the ridge were not visited. Park continues, however, “This breccia is flanked or overlain by the siliceous deposit on all sides of the Awana Flat. It is composed of a bluish-grey, shining, siliceous rock embedded in a matrix resembling a fine ash or tuff…. On the low spurs and ridges near the fantastically carved rocks overlooking the great Kaitoke basin this breccia material is seen to decompose in a deep, yellowcoloured, mealy, sandy clay. The origin of this breccia is evidently connected with some of the volcanic outbursts of andesitic matter which preceded the deposition of the great sinter deposit around this region.”
Whilst admitting that his visit afforded him but a cursory glance at the geology of this district, and that he had not read Park's statement in advance of it, the writer is at a loss to understand the location of this breccia. He would certainly doubt that it forms the “fantastic and grotesque” features of the ridges west of Awana Flat, and there is some uncertainty as to whether or not Park actually means that it does so in this locality, since he says that “This breccia is flanked or overlain by the siliceous deposit on all sides of the Awana Flat.” To the east and south-east of Awana Flat the topography indicates that the rhyolitic rocks give place to the andesitic, which are known to outcrop a very few feet below the level of the flat on the south-western slope to the Kaitoke Stream from the flat, and which form a wide strip along the east coast north from the mouth of that stream.
The writer did, however, observe a most curious obsidian “breccia”—possibly a true breccia, more probably only such in appearance, and in reality a weathered coarsely perlitic obsidianitic flow. This is exposed, lying hard upon the andesitic rooks (mentioned above) outcropping a very few feet below it, in a cutting of the track from Awana Flat towards Oroville, the site of the now disused mines in the Kaitoke Valley. The depth of the breccia actually exposed is insignificant, but it is possible that its thickness is locally considerable, and that it represents some phase of the breccia described by Park.
The obsidian discovered here explains the otherwise inexplicable occurrence of obsidian noted by McKay (1897) on Te Ahumata, which is, indeed, a further argument in favour of the view that the upper portion of that elevation—the sinter of McKay and Park—is largely rhyolitic. This mass rests on andesitic material which is exposed on the lower slopes of the Whitecliffs Range, and which, in altered form—propylite—is the country of the gold-silver veins (fide Park, 1897).
Petrography of the Rhyolites.
Petrographically the rhyolites examined from the acidic area north of Te Ahumata show little variety. In hand-specimen none showed any noticeable phenocrysts, and the majority, as already stated, have a very fine, regular, but somewhat sinuous fluxion-banding. The colours vary from greyish-white to pinkish. In thinnest section, using a microscope with good resolving-power, the banded varieties show up as very minutely micro-spherulitic. Phenocrysts are practically absent. Small druses of opal may show up in section, and in the field a few larger opal- or chalcedony-filled cavities were observed.
One section of a banded type shows dense somewhat axiolitic narrow dark bands, between which are clearer bands largely of very finely crystalline quartz. A few very small crystals of feldspar represent the only phenocrysts.
An Acid Dyke in the Andesitic Fragmentals.
Before leaving the subject of the rhyolitic rocks, mention must be made of a very striking dyke, apparently of rhyolite, forming a conspicuous feature of the landscape on the northern wall of the Awana Valley. It can be seen readily from the open country near the top of the ridge crossed by the inland or “bush” track from Harataonga Bay to Whangapoua. Seen from there at a distance of a little less than three-quarters of a mile it appears a great vertical wall, probably at least 100 ft. high, built of horizontal columns apparently of rhyolite, which outcrops near it to the south, and piercing andesitic conglomerates clearly visible in the adjacent bluffs. (See Plate XXV, fig 1.)
Petrography of the Dyke Complex in the Basement Sediments.
Having read Hutton's (1869) description of the Mine Bay area, the writer had anticipated that his own visit would yield him petrographic material of very great interest. He was not altogether disappointed, but found that practically all the dykes he examined were greatly affected by decomposition or alteration of some kind or another. This fact militates against the exact deciphering of some of the varieties.
The list of rock-types examined includes (1) pegmatite or granite-granophyre, (2) quartz-porphyry, (3) quartz-porphyrite and quartz-andesite, (4) porphyrites and andesites. Hutton's original list includes diorite, quartz-porphyry, and felstone. In a later paper (1889) he describes an elvanite, or dyke-rock showing quartz phenocrysts in a felsitic groundmass. Apparently he did not examine the rocks microscopically, and his identifications are therefore by no means sound.
Very few indeed of the rocks have escaped the prevailing alteration. This is exhibited in the conversion of feldspars to kaolin, often with calcite and quartz as additional products, and the hydration of biotite and ferro-magnesian minerals generally to chlorite, sometimes with the addition of calcite. The production, and often introduction, of calcite is most general, and strings of it seam the dykes and adjacent sediments. Pyrite is a further secondary mineral which is occasionally abundant. In several instances metallization has proceeded along the walls of dykes, producing small lodes in which the commonest minerals are chalcopyrite, sphalerite, galena, and pyrite. It is possible that the general alteration of the dyke-rocks is a result of the same processes as gave rise to the introduction of oreminerals, but it must be admitted that the wall-rock of the dykes appears to give no evidence of any of the changes expectable on that hypothesis.
1. Pegmatite or Granite-granophyre.
This is a curious rock forming an intrusion on the south-east wall of the copper lode at Miner's Head, and surprisingly free from signs of having been affected by the near passage of the solutions giving rise to the lode. The minerals are equidimensional, and form a mosaic reminiscent of that displayed by granulites Much of the rock shows graphic structure.
Fig. 1.—A conspicuous acid dyke intruding andesitic conglomerates on northern wall of valley of Awana Stream. (Photographed with short-focus lens from a point distant about three-quarters of a mile.)
Fig. 2.—Ramifying narrow dykes intruding shattered shale of oldermass, south shore of Mine Bay, Great Barner Island.
Fig. 3.—Closely folded fine-bedded sediments of oldermass, north end of Harataonga Bay, east coast of Great Barrier Island.
Fig. 1.—Graphic intergrowth of quartz and orthoclase in pegmatite. Boulder from conglomerate in oldermass, Mine Bay Creek. Crossed nicols. × 26.
Fig. 2.—Granulite with garnet (black, in centre) from conglomerate in oldermass, Mine Bay Creek. Crossed nicols. × 26.
Fig. 3.—Granulite from conglomerate in oldermass, Mine Bay Creek. Crossed nicols. × 26.
Fig. 4.—Hypersthene with resorbed borders in “second period” andeside, Kaitoke Valley. × 26.
Fig. 5.—Section of a typical “second period” andesite, showing a phenocryst of hypersthene and the characteristic fine groundmass. × 26.
Fig. 6.—Phenocrysts of zoned plagioclase and of quartz in quartz-porphyry from headwaters of Mine Bay Creek. Crossed nicols. × 26.
Fig. 1.—A porphyrite from the dyke complex of Mine Bay. Part of a large plagio clase crystal is recognizable, and the irregular nature of the coarse giound mass can just be discerned. Crossed mcols. × 26.
Fig. 2.—Sagenitic pseudomorph after biotite in porphyrite from Mine Bay area. × 150.
Fig. 3.—Quartz-miea-porphyrite from intrusion near old copper-mine. Phenocrysts of biotite and plagioclase can be seen. Crossed nicols. × 26.
Fig. 4.—A quartz phenocryst from a hornblende-biotite-quartz-porphyrite forming intrusion a little south of Miner's Head. The crystal shows a distinct corrosion border. Crossed nicols. × 26.
Fig. 5.—Pilotaxitic andesite from dyke in oldermass of Mine Bay Creek. Crossed nicols × 26.
Quartz forms about half the bulk of the minerals, though cryptoperthite is fairly plentiful along with orthoclase. The only other minerals are a little plagioclase and some flakes of biotite. It is possible that this rock is the “true dyke of granite” mentioned casually by Hector (1870) in his paper on mining in New Zealand.
There is some uncertainty in the writer's mind as to the exact classification of this rock; the structure is typical neither of pegmatites nor granophyres.
Several dykes of this type are recorded by Hutton (1869), as well as a number of an allied rock called by him “felstone,” and defined by Hector (1870) as a rock lacking phenocrysts, but otherwise similar to quartz-porphyry. Many of Hutton's quartz-porphyry dykes seem to be in reality porphyrites with quartz.
The quartz-porphyries so called by the present writer have rather finely granular groundmass and large phenocrysts of quartz and weathered orthoclase and plagioclase, with a few flakes of altered biotite. The groundmass seems largely feldspar, so that the classification is perhaps uncertain in the absence of chemical analysis. Dykes of this rock are rare; only three were found, all of them close together in the headwaters of Mine Bay Creek.
Many of Hutton's “felstone” dykes are very doubtfully acidic. The groundmass approaches the felsitic, but the phenocrysts, if present, are rare ones of feldspar. The alteration is so intense that exact identification is almost impossible. In the field these dykes may ramify in intricate fashion, as can be seen from Plate XXV, fig. 2.
The photomicrograph, fig. 6 of Plate XXVI, exhibits a typical quartz phenocryst, along with zoned plagioclase, in a quartz-porphyry forming a narrow finely banded dyke in the headwaters of Mine Bay Creek.
3. Quartz-porphyrites and Quartz-andesite.
The distinction between the terms “porphyrite” and “andesite” when applied to dyke-rocks is at best an artificial one. Those here classed as porphyrites show rather coarse feldspar phenocrysts in hand-specimen, along with prominent biotite or else hornblende, or chlorite pseudomorphs after those minerals; they have a coarse groundmass lacking the usual structures found in andesites. In the Great Barrier dykes these latter rocks are very typical representatives of their class; feldspar is their only common prominent phenocryst. Fig. 1 of Plate XXVII illustrates the coarse irregular structure of the groundmass of a type best classed as a porphyrite.
Porphyrites and andesites are by far the commonest of the intrusives; some contain quartz and some are without it. All show the prevailing alteration, though in a few instances this is not intense. Two good examples of comparative freshness are furnished by a quartz-biotite-porphyrite forming a massive intrusion near the adit of the old copper-mine, and by a wide intrusion in a small bay just south of Miner's Head, which is mapped by Hutton as a diorite, but is in fact a quartz-porphyrite, rich in both biotite and hornblende.
The majority of the quartz-porphyrites are types with altered biotite, usually with chloritic, sometimes with sagenitic, pseudomorphs after that
mineral. An excellent example of such a sagenite pseudomorph is afforded by fig. 2 of Plate XXVII.
In a section cut from a greatly altered porphyrite-like rock there are nests of chalcedony and some phenocrysts still recognizable as microperthite in spite of their being largely replaced by calcite and quartz. In the rather fine-grained groundmass there are laths of plagioelase, whilst a little of the quartz appears to be primary.
In the quartz-porphyrite from near the copper-mine the phenocrysts are mainly basic andesine, with a moderate number of flakes of biotite. The groundmass is fairly coarse, and consists mainly of plagioclase with abundant small shreds of biotite and a certain amount of quartz; it contains occasional radial structures. Occasional pseudomorphs after hornblende are recognizable. Fig. 3 of Plate XXVII illustrates a typical portion of this porphyrite, whilst fig. 4 portrays a quartz phenocryst in the hornblende-biotite type which has already been mentioned. In this latter rock the biotite is very fresh, and is perhaps in excess of the greenish-brown somewhat chloritized hornblende; a few large phenocrysts of quartz are visible, typically corroded, but andesine is again the most abundant phenocryst. There is quartz in the groundmass. Zircon, apatite, and iron-ore are present in small amount.
The only andesitic dyke-rock containing phenocrysts of free quartz outcrops in the bed of Mine Bay Creek about a mile above its mouth. It contains ilmenite with associated sphene.
4. Porphyrites and Andesites.
Porphyrites and andesites are amongts the commonest of the intrusive rocks represented. A mica type, always greatly altered, is the most prevalent of the porphyrites. The only other variety noted is a coarse highly feldspathic one, almost lacking in ferro-magnesian minerals.
The andesites are varied. Even when appearing fairly fresh macroscopically, all are found in section to be altered to a greater or less extent. Some are highly feldspathic, some noticeably pilotaxitic (see photomicrograph, fig. 5, Plate XXVII). Many are altered beyond recognition of variety. It is possible, however, positively to identify the following varieties: Micaandesite from a small tributary to Mine Bay Creek about half a mile above its mouth, augite-andesite from a prominent dyke at the north end of Harataonga Bay, and an andesite with brown hornblende from a dyke near the north head of Mine Bay. In the augite-andesite the structure of the groundmass is unusual, for the plagioclase laths are enwrapped pseudopoecilitically by a clear mineral resembling quartz.
Comparison with Intrusives of Coromandel Peninsula.
The presence of abundant porphyrites and andesites in the basement rocks of Great Barrier Island is another evidence of the close similarity of that area to the Coromandel Peninsula, where even greater variety is shown in dykes of the same petrographic character, which are intrusive especially into the Moehau series of pre-Jurassic age (Fraser and Adams, 1907, p. 22). Particularly on the western flank of the Moehau Range intrusions are both numerous and varied, but all are basic intermediate in character (Fraser and Adams, 1907, pp. 87–93).
It is impossible in Great Barrier Island to form any estimate of the age of the intrusions, or indeed to correlate the basement rocks themselves
with the divisions accepted by Fraser and Adams for those beds in the Coromandel Peninsula, but these authors adduce several arguments in favour of the view that many of the similar intrusions in the district referred to must have been pre-Tertiary (1907, pp. 88–89). There can be little doubt that those of Great Barrier Island are substantially contemporaneous with these latter.
Origin of Copper Lodes.
Unfortunately the writer, has not had the opportunity for a very close study of the copper deposits; the main lode has been worked out, and only a few remnants of the lode-material are visible here and there in the workings. Such information as is available makes Hutton's view of the origin of the deposit untenable, in spite of the fact that he had the advantage of examining the lode whilst mining operations were in progress. Hutton (1869) considered that it originated superficially as a breccia filling a surface fissure.
The present writer's view is that the lode is due to the metallization of an irregular shatter-zone trending approximately north and south. The solutions depositing the cupriferous material were almost certainly genetically related to the numerous porphyrite intrusions near by. This view has very weighty support from the presence of small veins of mixed sulphides—largely chalcopyrite with galena, blende, and pyrite—which are exposed actually on the walls of porphyrite dykes in old prospecting-drives on the north shore of Mine Bay, and up a rill entering Mine Bay Creek from the north about 15 chains up-stream from its mouth (the “New Lode” of Hutton's map).
In the deposit at Miner's Head the ore-minerals are mainly chalcopyrite with its oxidation products; these have been deposited in the crevices of the shattered vein-filling, which is predominantly a somewhat altered argillaceous rock. Hutton considered that the presence of fragments of “diorite” in the vein-filling showed that the intrusions were earlier than the lode, and therefore had no genetic relations to this latter. He apparently failed to appreciate the possibility that the intrusions are not all absolutely contemporaneous.
List of Literature cited.
Babtrum, J. A., 1920. The Conglomerate Band at Albany, Lucas Creek, Waitemata Harbour, Trans. N.Z. Inst., vol. 52, pp. 422–30.
Cotton, C. A., 1916. The Structure and Later Geological History of New Zealand, Geol. Mag. (n.s.), dec. 6, vol. 3, pp. 243–49 and 314–20.
Fraser, C., 1910. The Geology of the Thames Subdivision, N. Z. Geol. Surv. Bull. No. 10 (n.s.).
Fraser, C., and Adams, J. H., 1907. The Geology of the Coromandel Subdivision, N.Z. Geol. Surv. Bull. No. 4 (n.s.).
Hecutor, J., 1870. On Mining in New Zealand, Trans. N.Z. Inst., vol. 2, pp. 361–84.
Hutton, F. W., 1869. Report on the Geology of the Great Barrier Island, Rep. Geol. Explor. during 1868–69, pp. 1–7.
—– 1889. The Eruptive Rocks of New Zealand, Jour. and Proc. Boy. Soc. N.S.W., vol. 23, pp. 102–56.
McKay, A., 1897. Report on the Silver-bearing Lodes of the Neighbourhood of Blind Bay, Great Barrier Island, Auckland, N.Z. Parl. Paper C.-9, pp. 75–80.
Park, J., 1893. On the Occurrence of Granite and Gneissic Rocks in the King-country, Trans. N.Z. Inst., vol. 25, pp. 353–62.
—– 1897. The Geology and Veins of the Hauraki Goldfields, New Zealand, Trans. N.Z. Inst. Min. Eng., pp. 1–105.
Sollas, W. J., and McKay, A., 1905. Rocks of Cape Colville Peninsula, vols. 1 and 2.